Is there a bpy function to convert from blender units to Imperial?
I found several internal conversion code in the source, like one in view3d_ruler.c for bpy.ops.view3d.ruler(), or in drawobject.c for measurement stats, none of them with direct Python interface. No reference to it, too, in the API manual.

CMIIW, but I think we'll have to write one. All distributed addons so far, like Measure Panel, also write their own conversion function, and it looks like the aforementioned addon have several function we can use directly:

import space_view3d_panel_measure as pm

uinfo = pm.getUnitsInfo()    
print(pm.convertDistance(2, uinfo)) # prints 2.0m, 2.0 BU, or 6.56168'
                                    # depending on selected unit system.